Variable optical attenuators (VOAs) serve two primary purposes in optical transmission systems: first, it is often necessary to reduce the power level of an incoming optical signal to match its intensity to the optimum operating level of a receiver component. Second, an optical attenuator is also an essential element of any equalization component, which aims at adjusting the power in the optical channels to correct signal distortions experienced throughout the transmission. The adjustment of the power level of optical signals should be done with high accuracy, repeatability and reliability. Furthermore, the present communications market requires these devices to be fabricated at low cost. Solid-state devices designed and fabricated using Planar Lightwave Circuits (PLC) technology, with no moving parts, can fulfill these demands.
A 1×1 Mach-Zehnder interferometer (MZI), one of the most successful and useful structures built in PLC technology, can be employed as a VOA with very wide dynamical attenuation range, from total transparency (except for a small insertion loss) in its passive state, to absolute active suppression (in some cases beyond 30 dB) of the incoming signal. Nevertheless, a VOA based on an MZI configuration (as being used today) usually suffers from a serious drawback, namely its polarization dependence.
In a simple prior art embodiment, a 1×1 MZI is based on a Y splitter and a Y combiner facing each other along the main propagation axis, and two symmetric arms connecting the inward branches of the splitter and the combiner. The Y-splitter splits the input signal into two symmetric, coherent and in-phase signals, each carrying 50% of the incoming energy. The dynamical performance of the MZI is controlled through active elements located on at least one, and preferably on both of the internal arms. Each active element can change the refractive index of the arm upon which it is located and, therefore, can induce a difference in the optical lengths of the two (otherwise symmetric) arms. Consequently, the active element introduces a controlled phase shift between the coherent signals propagating in the two arms. The combiner gathers the symmetric projection of the two shifted signals at the output of the MZI, and radiates the antisymmetric projection as losses. This yields the required attenuation.
In other prior art 1×1 MZI embodiments, the Y splitter, the Y-combiner, or both, can be replaced by 2×2-couplers (either directional couplers, adiabatic couplers or any other coupler, for example, the use of two directional couplers in a MZI configuration as a variable attenuator is described in UK Patent Application GB 2 187 858 A) to form a 1×2, 2×1 or 2×2 configuration with one input waveguide and/or one output waveguide used as idle ports. In configurations with an idle output port, the discarded projection of the actively shifted signals is not radiated, but transmitted to this idle port.
In certain embodiments, the internal arms of the MZI may also have non-equal optical lengths (e.g. a MZI with directional couplers that transmits the signal in bar configuration in its passive state). These asymmetric embodiments have, in general, higher wavelength-dependent losses at low attenuation over broad wavelength bands. In order to reduce power consumption, it is usually advisable that the passive operational state yields maximal transparency (zero attenuation).
Nonetheless, in each of these prior art embodiments of an MZI structure, the VOA usually evidences a severe problem of polarization dependent loss (PDL), especially at high attenuation values. This happens because the phase of the two different polarizations, TE and TM, is shifted differently at the internal arms, due to fabrication process-related effects such as material stresses, inhomogeneity, etc, or due to intrinsic asymmetry in the waveguide cross-section (e.g. a waveguide width different from its height). The higher the attenuation, the more severe the problem becomes: while PDL is typically in the range of a small fraction of a dB when the attenuation is lower than 3 dB, it grows monotonically up to several dB units at high attenuations (5 dB and above).
Conventionally, a series of two or more concatenated MZIs in a tandem is used in order to achieve a higher attenuation range than that of a single element. When two MZIs are placed one after the other, the attenuated signal at the output of the first MZI enters subsequently into the second MZI as an input signal, and then undergoes a second attenuation process. The two MZIs are normally separated by a few millimeters of a straight waveguide that supports only a single optical mode, which guarantees that unwanted optical power, which was actively projected by the first MZI onto a high-order mode, is completely radiated and does not reach the input of the second MZI. The two concatenated MZIs can provide an attenuation range wider than 50 dB, twice the width of the attenuation range provided by a single MZI. However, this VOA, when operated in the conventional way, suffers from the same severe PDL problem affecting the VOAs based on a single MZI.
Therefore it would be highly advantageous to have a method of designing and operating a VOA built in PLC technology and desirably based on the well-known MZI structure, which can provide a very wide operational attenuation range and very low polarization dependence over the whole range of operation.